Damaging Long-period Ground Motions
from the 2003 Mw 8.3 Tokachi-oki, Japan
Earthquake
Kazuki Koketsu 1 , Ken Hatavama", Takashi Furumura 1 , Yasushi
Ikegami 1,3, and Shinichi Akiyama 3

INTRODUCTION
Although most seismic damage occurs within 10-20 km ofan event, long-period waves can cause destruction at muchgreater distances. The long-period component of seismic
ground motion generated by earthquakes attenuates more
slowly with distance than higher frequencies and so persists tomuch greater ranges. Large-scale structures such as tall buildings and big tanks can resonate with the long-period groundmotion because their own natural periods are in the same frequency band. The severity ofshaking can be such that panic is
produced for people in high-rise buildings from earthquakes
several hundreds of kilometers away (Pan, 1995; Pan andSun, 1996). Such long-period ground motions can even bedamaging in some circumstances; the worst example occurredwith more than 20,000 fatalities in Mexico City at a distanceof 400 km from the 1985 Michoacan earthquake (Beck andHall, 1986).
A further example was provided by Tomakomai, a coastal
city on the island of Hokkaido in northern Japan, which
was hit by long-period ground motion from the Tokachi-oki
earthquake 250 km away on the morning of 26 September
2003 (local time = UT + 9 hours). This M w 8.3 earthquake was the largest event in the world in 2003 (National
Earthquake Information Center, 2004), but damage was notextensive because the location is offshore. No one was killed,
although two people fishing during the tsunami are missing.
We note that no intensities larger than 6- were observed on
the JMA scale, where the highest ranks are 7 and 6+ (Japan
Meteorological Agency, 2003). The intensity at Tomakomai
was only 5-, which corresponds to no structural damage tohouses. Many oil tanks were damaged by the long-period
ground motions, however, and two of them caught fire
(Coffin and Hirata, 2003), as shown in Figure 1. This article
will discuss these damaging long-period ground motions and1. Earthquake Research Institute, University ofTokyo, Japan
2. National Research Institute of Fire and Disaster, Japan
3. CRC Solutions, Tokyo, Japan

our preliminary analyses. The earthquake itself has been discussed in more detail in the special section of Earth, Planets
and Space 56(3) (e.g., Koketsu et al., 2004).

OBSERVATIONSTomakomai is located in the center of the Yufutsu sedimentary basin facing the Pacific Ocean to the south (Figure 2).
We are able to combine observations from the nationwide
seismometer arrays K-NET (Kinoshita, 1998) and KiK-net(Aoi et al., 2000) to characterize the nature of the groundmotion. Within the sedimentary basin at stations HKD129
and IBUH05 there are very clear long-period componentsof ground motion with long duration. A shorter long-period
wave train is also found at station HDKH04 near the eastern edge of the basin (Figures 3A and 3B). Particle motion
analysis (Figure 3D) helps identify the changing nature of the
wave trains. The initial short-period part of the seismogramat HDKH04 consists primarily of S waves, since the groundmotion is polarized in the radial (east-west) direction fromthe earthquake source of low-angle reverse faulting. This isfollowed by a couple of cycles of waves with rather long periods, as indicated by the orange underbars in Figures 3A and
3B. These particle motions are also polarized in the radial
direction, so they can be identified as surface Rayleigh waves.
These waves can even be found at HDKH06 outside thebasin, although they are contaminated by short-period reverberations. This means that these Rayleigh waves must havebeen generated in the source region or during their passage
to HDKH06.
The second segment of long-period ground motion, indicated in purple in Figures 3A and 3B, is polarized in the transverse direction with oval orbits (Figure 3D). This portion of
the seismograms consists mainly of surface Love waves andsome other waves associated with the 3D complexity of thebasin structure. The maximum ground velocity of the surface waves on the north-south seismograms is larger at station HKD129 in Tomakomai (32.6 cm/s) than at HDKH04(30.4 cm/s), even though the latter station is 40 km closer
to the earthquake. The duration of the long-period ground

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67

.... Figure 1. Aerial view of the fire in the oil tank damaged by long-period ground motions from the 2003 Tokachi-oki, Japan earthquake(courtesy of Dr. Hiroshi Koseki, National Research Institute of Fire and Disaster, Japan).

Altitude

42

0

2000
1200
900
600
300
150
75

km

a a

50
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142

0

143

*
0

144

.... Figure 2. Index map of the Tomakomai and Hidaka regions in Hokkaido, Japan. The black triangles denote seismometer stations in andaround the Yufutsu basin, where strong ground motions from the 2003 Tokachi-oki earthquake were observed. The blue star indicates theepicenter ofthe earthquake.

68 Seismological Research Letters Volume 76, Number 1 January/February 2005

A

(km)

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200 (s).A. Figure 3. Sections of (A) north-south and (B) east-west velocity seismograms observed at the four stations. The traces consist of S
waves, Rayleigh waves (orange underbar), and induced Love waves (purple portion). Panels Cand 0 display the velocity spectra and horizontal particle motions at HDKH05 (blue), HDKH04 (green), HKD129 (red), and IBUH05 (black).

motions is also longer at HKD 129 by about 100 s. Similar
behavior to HKD129 is seen at IBUH05, which is located on
the other side of the Yufutsu basin.
The frequency spectrum of horizontal ground velocity at
HKD 129 is dominated by components in the period range
from 4-9 s (red curve in Figure 3C), whereas the main contribution at HDKH04 lies in the 4-6 s band (green curve).
Although the seismogram at HDKH06 shows no obvious
long-period arrivals, we can find modest peaks with these
periods in the ground velocity spectra observed there (bluecurve). The high-amplitude peaks correspond to the Rayleighwaves, which travel via HDKH06 to the basin edge near
HDKH04 and are then converted to basin surface waves withsimilar or longer periods (Kudo, 1980; Koketsu and Kikuchi,
2000). The Love waves with periods from 6-9 s are produced
at the edge of the basin and develop further as they propagate
across the basin (Frankel et al., 1991; Hatayama et al., 1995;
Frankel et al., 2001). The basin surface waves create the 4-6 s
peak in the spectrum at HDKH04, which extends after further propagation to HKD129 into the much broader zone
shown in light gray in Figure 3C.

SIMULATIONS
The above interpretations are supported by preliminary 3D
numerical simulations of the ground motion. A 3D structuralmodel for the region outlined in blue in Figure 2 was constructed exploiting the results of reflection and microtremor
surveys (Asano etal., 1989), together with the surface and seabed topography (Geographical Survey Institute, 2004; Japan
Oceanographic Data Center, 2004). The sea to the south of
the Yufursu basin was included in the model with zero rigidity(Stephen, 1988). The ground-motion simulations were thencarried out using a finite-element method with a voxel mesh
(Koketsu et al., 2004) and excitation by an input plane wave
specified by the seismic motion observed in the borehole siteat HDKH06, located just outside the region. To account for
amplification at the free surface, the borehole motion amplitude was reduced by one half.
The pattern ofhorizontal ground motion from the numerical simulations is shown in Figure 4, superimposed on thesurface topography. The first snapshot at 4:51: 17.52 (70.10 s
after the earthquake origin time of 4:50:07.42) shows themoment when the S waves emerge into the central part of the

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59

o
4
8
12

16

20
40
cm /s

... Figure 4. Snapshots of the distribution of simulated horizontal ground velocities (1-4).The significant wavefronts are specified with thelabels "S-wave", "Rayleigh waves 1 & 2", and "Lovewaves 1 & 2."
basin. In this frame, the Rayleigh wave from the source region
is converted to a basin surface wave at the eastern basin edge,
which lies under the sea. Another basin surface wave is converted along the boundary berween the basin and the Hidaka
Mountains ("Rayleigh wave 2" in frame 2 at 4:51 :20.18) .
These rwo Rayleigh waves collide at the sourhern coast of the
Yufutsu basin and form a large-amplitude disturbance withan angular margin as can be seen in the third snapshot at
4:51:22.46. Both of the Rayleigh waves contributed to the
wavefront at the cusp near the coast, close to Tomakomai, sothat the east-west component at HKD129 shows well developed long-period ground motions, as indicated by the orange
underbar in Figure 3B.
We can identify short-period arrivals behind the 5 andRayleigh waves in the HDKH06 seismograms (Figures 3A
and 3B). This 5 coda hits the basin edge and produces thebasin Love waves. The first Love wave train is produced at theundersea part of the basin edge as shown in snapshot 2. In asimilar fashion to the converted Rayleigh wave in the basin,
the second Love wave train was produced at the basin-mountain boundary ("Love waves 2" in snapshot 3). The rwo Lovewaves interact with each other, and new Rayleigh waves canbe generated from interconversions of surface-wave modes inthe 3D basin structure. The combination ofall these processes
results in whole-basin reverberation with long-period groundmotion, as shown in snapshot 4, which matches the behavior
seen on the HKD129 and IBUH05 seismograms in Figures
3A and 3B.
The second Love wave train can be confirmed by theobserved particle motions. Figure 5 displays the snapshot forthe period from 70- 80 s after the origin time . The motions
at stations IBUH03 and ATSUMA near the basin-mountainboundary are polarized in the northwest-southeast direction , whereas the orbit at station HKD126 shows only thetransverse motion of the ordinary Love wave. We conducted
a temporary array observation of the aftershocks in October
2003. Its result also shows particle motions polarized in the
direction along the boundary berween the Yufutsu basin andthe Hidaka Mountains.

OIL TANKS
The strong long-period ground motion can have destructive
impact on large structures whose natural periods lie in the

70 Seismological Research Letters Volume 76, Number 1 January/February 2005

km

70 - 80so

1020

142 00'141 30'• Figure 5. Distribution ofparticle orbits ofthe long-period ground motions inthe Yufutsu basin and itsextensions during the period from70-80 s after the earthquake. The motions polarized inthe direction along the basin-mountain boundary were observed atstations IBUH03
and ATSUMA.
same period band. In the case of Tomakomai, a number ofoil tanks were damaged . We use the recorded ground motion
to investigate the behavior of an oil tank that was damaged
during the earthquake and caught fire two days later (Figure
6A and Figure 1), by using finite-element analysis of fluid
sloshing with solid voxels and fan-shaped liquid elements(Figure 6B). The tank had a capacity of about 30,000 m 3 andat the time of the earthquake contained 25,000 m 3 of volatile
chemicals called naphtha (Hatayarna et al., 2004) . The firststep was to solve for the eigen periods of vibration of the tank;the period 7.12 s for the fundamental mode of the sloshinglies in the middle of the period band for the large-amplitude
ground motion at Tomakomai (red spectrum in Figure 3C) .
The naphtha in the tank would therefore resonate with theground motion, producing large sloshing waves.
The second step was to extend to a full dynamic analysis with a fixed foundation subject to the ground motion
observed at station HKD 129, located very close to the tank

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350 (s)

o

-3

+3m

04:51:42.5,

04:52:01.5

04:52:02:0

04:52:03.0.A. Figure 6. Snapshots ofthe distribution of simulated naphtha surface motions (1-6) . Panels Aand Bshow a photo ofthe fire inthe tank
and an FEM model of the tank, respectively. The red arrow points to the position of the highest sloshing wave. Panel Cdisplays the time
history ofthe sloshing atthis position.

(Figures 3A and 3B). The computed surface motion for thenaphtha is displayed in Figures 6-1 to 6-6. The arrival of thebasin Rayleigh waves provides the first long-period impact on
the tank. Since this wave is polarized in the east-west direction, the naphtha initially vibrated along this direction, as
shown in snapshot 1. The second impact was then generated
by the basin Love waves, which were mostly polarized in thenorth-south direction. The initial east-west sloshing decayedrather slowly, however, and the ground motion excited by theLove waves included some east-west motions arising from the3D basin structure. Accordingly, snapshots 2 and 3 show anorthwest-south east motion of the naphtha surface.
The sloshing motion builds up to an amplitude of2.68 m
around the time of snapshot 3. Position C of the maximum
height reached is shown with the red arrow in Figure 6B. The
long-period ground motion continued for more than two
minutes after the highest sloshing, with a counterclockwise
rotation as shown in the part icle motion diagram of Figure
3D . Consequently, the mot ion on the naphtha surface wasalso rotated counterclockwise, as shown in snapshots 4 to 6.
Figure 6C shows the time history of the sloshing wave heightat point C. Since the damping factor of the naphtha is as
small as 1%, the wave height decreased very slowly and large
sloshing continued for a long time.

72 Seismological Research Letters

CONCLUSIONSIn September 2003 the city of Tomakomai in Hokkaido,
Japan suffered substantial damage to large oil tanks fromlong-period seismic waves generated by the 2003 Tokachioki earthquake , more than 250 km away offshore. The disturbance in the large sedimentary basin within which thecity lies is characterized by combining observations and 3D
numerical simulation s. A very important contribution camefrom surface waves stimulated in the basin, either directly or
by conversion at the margins, which led to large amplitudes
and duration of shaking. Fluid sloshing in a damaged oil tankoccurred with a period close to that of the dominant groundmotion, producing meters of displacement and a long fluid
oscillation that contributed to the destruction of a floatingroof. The naphtha was then exposed to the air and became
flammable. An unknown cause finally ignited the subsequent
fires.
The 2003 Tokachi-oki, Japan earthquake reproduced
the phenomenon of damage at a distance seen in the 1985
Michoacin earthquake and demonstrates that a magnitude
8 earthquake can generate damaging long-period groundmotions in a distant sedimentary basin. Since no tall buildings were in Tomakomai, the tragedy in Mexico City was

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fortunately not fully reproduced. The event does provide atimely reminder of the long-range impact of earthquake
damage, however. A future San Francisco earthquake willcause not only tragedy in the Bay Area but might also have asevere impact on the deep sedimentary basin underlying Los
Angeles. Heaton et ale (1995) have already warned us that alarge earthquake just beneath a metropolitan area can severely
damage structures with long natural periods. Distant earthquakes with magnitudes 8 and above are also potentially damaging for these structures. E~

ACKNOWLEDGMENTS
We thank Prof Brian Kennett (ANU) and Dr. Susan Hough(USGS) for careful reviews. The National Research Instituteof Earth Science and Disaster Prevention and JMA provided
us with the strong-motion records. This study was supportedby the Special Coordination Funds for Promoting Science andTechnology, and the Special Project for Earthquake Disaster
Mitigation in Urban Areas from MEXT ofJapan.

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Earthquake Research Institute
University ofTokyoJapan

koketsu@eri.u-tokyo.ac.jp(KK, T.E, Y:L)
National Research Institute ofFire and Disaster
Japan

(KH.)
CRC Solutions
Tokyo
Japan

(Y:L, S.A.)

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